"Approximately one-third of women older than 65 fall each year, and six percent sustain a fracture as a result of the fall. In addition, fear of falling is a major problem in older people," according to background information in the study, conducted by Dr. Richard L. Prince, of the Sir Charles Gairdner Hospital in Nedlands, Australia, and colleagues.

Because the body produces vitamin D in response to sun exposure and because this study was conducted in a sunny climate, the researchers recruited women with blood vitamin D levels below the median for the area (24 nanograms per milliliter). All the 302 women in the study were aged between 70 and 90 and had a history of falling in the previous year.

For the study, all the women received 1,000 milligrams of calcium citrate per day. Half of the women also received 1,000 international units of vitamin D2 (ergocalciferol) while the other half received a placebo.

During the study period, 80 women (53 percent) in the vitamin D2 group and 95 women (62.9 percent) in the control group fell at least once. After accounting for other factors, the researchers concluded that taking vitamin D2 reduced the risk of having at least one fall by 19 percent.

"When those who fell were grouped by the season of first fall or the number of falls they had, ergocalciferol treatment reduced the risk of having the first fall in winter and spring but not in summer and autumn, and reduced the risk of having one fall but not multiple falls," the study authors wrote.

The vitamin D supplementation, added to high calcium intake, was associated with a 23 percent reduced risk of falling in winter/spring -- to the same level as summer/autumn.

"It is interesting that the ergocalciferol therapy effect was confined to those who were to sustain one fall but not those destined to have more than one fall," they wrote. "Older people who fall frequently tend to have more risk factors for falling, including greater degrees of disability and poorer levels of physical function."

Boosting vitamin D levels may not be sufficient to prevent falls in these people, the authors said.

The study was published in the Jan. 14 issue of the journal Archives of Internal Medicine.